Polymer/pigment concentrates have been used for many years, both for economic reasons and for increasing the range of colors obtainable with polymers that are difficultly dyeable. Typical prior art includes the following.
U.S. Pat. No. 3,275,591 teaches the use of impact devices to partially break down pigment agglomerates in blends of pigment with polypropylene powder of undefined particle size, and exemplifies pigment concentrations of 0.5% in the concentrate.
U.S. Pat. No. 2,497,346 teaches that tumbling of fine polymer particles with dry pigment results in an unacceptable molded product because of dullness and shreaks, and that it is desirable to modify that process of spraying the mixture with just-sufficient solvent to dissolve the surface of each (nonporous) polymer particle without consolidation of the particles. It further teaches that, in this solvent process, the finer the polymer particle size the more uniform is the final color obtained and that for economical reasons it is preferable to use particles larger than 100 mesh (which has openings of 0.147 mm or 147 microns). All its examples show pigment concentrations of less than 0.5%.
U.S. Pat. No. 3,600,354 teaches that in the preparation of polymer/pigment concentrates it is desirable to (i) treat solid polymer particles to roughen and fissure their surfaces and reduce them to finely divided form, (ii) mix the treated polymer with pigment to incorporate the pigment into the fissures, and (iii) heat the mixture sufficiently to sinter the particles. The patent further teaches that it is not generally sufficient to use small chips since these will have smooth surfaces to which the polymer will not adhere sufficiently. It also teaches that although the size of the particles is not critical, best results have been obtained with sizes in the range of 10.sup..sup.-5 to 7.times.10.sup..sup.-1 mm.sup.3, preferably of 10.sup..sup.-4 to 10.sup..sup.-1 mm.sup.3. In all the examples at least 85 weight percent particles had a maximum chord in excess of 20 microns. Further, the particles were typically in the form of long splinters or chopped fibers.
German Pat. No. 1,454,755 teaches that, for non-fiber applications, a particle size of 100 to 200 microns is adequate in the preparing of a polyethylene/pigment concentrate by dry ball milling (Example 1); and that an average particle size of 50 microns is adequate for polyvinyl chloride (Example 2).
Out of the aforementioned prior art, only U.S. Pat. No. 3,275,591 actually exemplifies polypropylene/pigment concentrates; and it fails to indicate particle size and fails to show concentrates at above 0.5% concentration. None of the forementioned prior art, whether relating to polypropylene or other polymers, either exemplifies the use of particles having an average maximum diameter of less than even 50 microns, or suggests that the presence of a small weight percentage of larger particles is harmful, but rather teaches the opposite.